1. Field of the Invention
The present invention relates to a semiconductor image position sensing device, and more particularly to a semiconductor image position sensing device for sensing a position of a spot image at high speed. Especially, the invention relates to a semiconductor image position sensing device used in a sensor for determining an optical position in a variety of automation systems or a variety of optical measuring systems and suitable for realizing a high-speed and simple sensing system or measuring system.
2. Description of the Related Art
Heretofore, a semiconductor image position sensitive device has been known as a sensor for detecting simply a spot image position at high speed.
In general, a semiconductor image position sensitive device is composed of a photoelectric layer, a dividing resistance layer laminated on the photoelectric layer, and signal current output terminals connected to the dividing resistance layer. Such a semiconductor image position sensitive device is constituted on the basis of a basic principle that when light irradiated from a spot is input to the photoelectric layer, a photoelectric current is generated in the photoelectric layer, the photoelectric current thus generated in the photoelectric layer is allowed to flow into the dividing resistance layer, whereby the photoelectric current distributed in response to a resistance value between an inflow portion of the photoelectric current in the dividing resistance layer and the signal current output terminals is settled, and a center position of incident light into the photoelectric layer is calculated based on an electric current value output from the signal current output terminals.
A conventional semiconductor image position sensitive device constituted on the basis of the above described basic principle will be explained in detail herein by referring to the accompanying drawings.
Namely, FIG. 1 is a conceptual view showing the structure of a conventional semiconductor image position sequence device. FIG. 2 is a conceptual diagram of an equivalent circuit exhibiting a principle of the calculation for sensing an image position in the semiconductor image position sensitive device of FIG. 1. The semiconductor image position sensitive device comprises a P-type semiconductor layer P, and insulator layer I laminated on the bottom side of the P-type semiconductor layer P, an N-type semiconductor layer N laminated on the bottom side of the insulator layer P, a resistance layer Rp for calculating an image position and which is laminated on the surface side of the P-type semiconductor layer P, a signal current output terminal A as well as a signal current output terminal B formed on the opposite ends of the resistance layer Rp on the surface side thereof, and a bias terminal C formed on the bottom side of the N-type semiconductor layer N at the central portion thereof.
In the above described semiconductor position sensitive device S, a photoelectric layer S is formed from the P-type semiconductor layer P, the insulator layer I, and the N-type semiconductor layer N, while the dividing resistance layer is formed from the resistance layer Rp.
In such semiconductor image position sensitive device as described above, when light L is irradiated from the surface side of the resistance layer Rp, photoelectric current generated in the photoelectric current layer S at an incident position of the light L flows into the resistance layer Rp, the photoelectric current thus flowed into the resistance layer Rp is distributed in response to a resistance value defined between a position at which the photoelectric current flowed into the resistance layer Rp and the signal current output terminals A and B, whereby output signal currents IA and IB are output from the signal current output terminals A and B, respectively (see FIG. 2).
In this case, when it is supposed that resistivity of the resistance layer Rp is constant, the resistance value is proportional to a distance defined between the position at which photoelectric current is flowed into the resistance layer Rp and the signal current output terminals A and B, so that information X at an incident position of the light L (being equivalent to a ratio of dislocation from the central position of the resistance layer Rp) is determined by an equation (1):x=(IA−IB)/(IA+IB)  (1)
In the meantime, the photoelectric current layer is continuous, and the resistance layer Rp being a dividing resistance for calculating an image position is formed as a thin film superposed on the photoelectric layer S in the semiconductor image position sensitive device shown in FIGS. 1 and 2.
However, it is not so easy that the resistance layer Rp being a dividing resistance for calculating an image position is formed stably as a uniform thin film having a predetermined resistivity, and as a result, such resistivity cannot be made constant, whereby a distribution of the resistivity becomes scattered. Thus, there is a problem that the scattering becomes a factor of an error in sensing for image position.
In order to solve such a problem as described above, a semiconductor image position sensitive device of separate photoelectric device type is devised wherein a photoelectric layer is fabricated as a separate photoelectric layer of a split structure separated into plural sections being independent of a dividing resistance layer, while the dividing resistance layer is fabricated stably as a constriction resistance at a position away from the separate photoelectric layer, and photoelectric currents generated in the split photoelectric layer having a structure which has been separated and split individually into sections are allowed to flow condensedly into positions corresponding to the dividing resistance layer.
FIG. 3 is a conceptual diagram of an equivalent circuit exhibiting a principle of such semiconductor image position sensitive device of a separate photoelectric device type as described above.
In FIG. 3, reference character Sg designates a separated photoelectric layer in the semiconductor image position sensitive device of a split photoelectric device type. According to the semiconductor image position sensitive device of a split photoelectric device type as described above, a resistance layer Rp can be stably fabricated as a dividing resistance for calculating an image position Thus, errors in sensing an image position are allowed to decrease, so that it is possible to improve stability in sensing an image position.
Furthermore, a photoelectric current generated by irradiating the light L in any structure in any semiconductor image position sensitive device as described above shown in FIGS. 1 through 3 is output from the signal current output terminal A as an output signal electric current IA, while it is output from the signal current output terminal B as an output signal current IB (see FIGS. 2 and 3). Accordingly, when a calculation is made on the basis of the equation (1) by applying the output signal currents IA and IB, it becomes possible to calculate a position of spot image by means of an analog arithmetic circuit at extremely high-speed.
Meantime, a gravitational position of all the light L input to a sensing region of light is sensed, but not the brightest point of a spot image in a semiconductor image position sensitive device based on the principle applying the above described equation (1). For this reason, it has been pointed out that there is such a problem that when a noise light such as background light occupying a large area though brightness is not high exists in a wide region in a peripheral section of an objective spot image, a significant error appears at a sensing position of light as a result of influence of noise light such as background noise.
In other words, if no background light exists in a wide region extending over the peripheral part around the objective spot image, a distribution of photoelectric current based on the light L derived from the spot image is as shown in FIG. 4(a). However, when noise light such as background light exists, a photoelectric current based on the noise light is also generated in a photoelectric layer S (separate photoelectric layer Sg), so that the resulting photoelectric current based on noise light such as background light is superposed on the photoelectric current based on the light L derived from the spot image, and hence, a distribution of photoelectric current becomes as shown in FIG. 4(b).
More specifically, when output signal currents IA and IB obtained from a photoelectric current onto which has been superposed noise light are applied to the equation (1) in case of existing noise light, a position of the spot image is calculated. As a result, such position of the spot image is biased towards a direction of gravitational position of noise light, so that there is a problem of generating a remarkable error in sensing the position.
When a further specific explanation is made in this respect, a photoelectric current generated in response to the light derived from a spot image is distributed to be output in accordance with a resistance value between a flowing-in position and output terminals because of presence of a dividing resistance in a semiconductor image position sensitive device, and when electric current values of the photoelectric currents which have been thus distributed to be output (output signal currents IA and IB) are calculated, a central position of the incident light L is determined.
For this reason, not only a photoelectric current generated in response to irradiation of the light L derived from a spot image as a signal to be sensed, but also a photoelectric current produced from noise light is reflected with respect to the output signal currents IA and IB in the case where noise light such as background light exists.
In order to avoid influence of such noise light, such a manner that a spot to be sensed is flashed on and off, an output in the case where the spot is flashed off is subtracted from an output in the case where the spot is flashed on, whereby influence of background light is removed has been heretofore applied.
However, the above described manner can be applied in only the case where spot can be flashed on and off, besides the case where noise light does not depend upon flashing on and off of the spot. In this respect, there is no effect with respect to reduction of errors due to noise light produced by irradiation of the light L derived from the spot.
In general, a density of a photoelectric current generated by noise light such as background light is considerably lower than that of a photoelectric current generated by irradiation of the light derived from a spot. However, since an area of incidence in noise light towards a photoelectric layer is remarkably wider than that of light derived from a spot towards the photoelectric layer, contributions upon the output signal currents IA and IB of the noise light are unable to disregard as a whole.
Accordingly, a position of gravity obtained by calculating the output signal currents IA and IB is dragged by a gravity of noise light such as background light, so that such a value which is deviated from a primary position of the spot is obtained. Thus, there is a problem wherein an error in case of sensing an image position becomes remarkable.
In this respect, since noise light such as background light is averagely distributed within a sensing region of a semiconductor image position sensitive device in general, a gravitational position of output signal currents IA and IB derived from noise light such as background light is in the vicinity of a central portion of a sensing region, so that a position for sensing an image is dragged by such result as described above, whereby the resulting value becomes the one which deviates towards the central portion of the sensing region.